The present invention relates to a radar system.
Some recent radar systems adopt an arrangement obtained by combining MDF (Multiple Doppler Filters) processing and LOG/CFAR (Constant False Alarm Rate) processing. This arrangement serves as a means effective for detecting a target signal from an aircraft, a ship, a vehicle or the like, and removing or suppressing an unnecessary signal, such as ground clutter (power reflected by the ground surface, e.g., mountains, woods and forests, fields, buildings or the like), sea clutter, weather clutter (reflected power caused by a meteorogical phenomenon such as a rain cloud and snow), and so on.
FIG. 1 is a block diagram showing a conventional radar system having such an arrangement. Referring to FIG. 1, a transmitter 1 and a receiver 4 are connected to an antenna 3 via a circulator 2. A transmitted pulse output from the transmitter 1 is input to the antenna 3 via the circulator 2 and radiated as short pulse radio waves. After the radio waves are reflected by a target and received by the antenna 3 again, they are input to the receiver 4 via the circulator 2. The receiver 4 mixes the received radio waves with a transmission frequency component, detects a frequency component corresponding to a difference between the transmission frequency and the received waves, and supplies the frequency component to an A/D converter 5. The A/D converter 5 converts the difference frequency component into a digital signal and supplies it to a canceller 6. When steady-state homogenously distributed signals are present, the canceller 6 discriminates them as a fixed clutter and cancels them out. An output signal from the canceller 6 is supplied to Doppler filters 7A to 7N.
Each of the Doppler filters 7A to 7N extracts only a signal component within its passband designed in advance. This is based on the following principle. More specifically, various types of targets and unnecessary background signals have inherent relative speeds with respect to the radar. Therefore, signals received by the radar from these background signals have inherent Doppler frequency components. When an MDF consisting of a plurality of Doppler filters having different passbands is provided, these reflected signals can be separated by utilizing the differences in Doppler frequency.
The target and clutter signals separatively detected by the respective Doppler filters 7A to 7N are input to CFAR processors 8A to 8N connected to the output terminals of the Doppler filters 7A to 7N, respectively, and are subjected to statistical normalizing processing in terms of amplitude. The principle of LOG/CFAR is as follows. Generally, a clutter signal spreads spatially uniformly since its respective reflection points that contribute to reflection exist in an area sufficiently larger than a radar range resolution defined by the pulsewidth of the radar. In contrast to this, a target signal is a point target since the physical size of a target is usually equal to or less than the radar range resolution. As a result, the clutter signal is sufficiently suppressed by LOG/CFAR processing, i.e., statistical normalizing processing, in terms of amplitude in the range direction, while the target signal is not, in principle, influenced by LOG/CFAR processing. In particular, it is theoretically accepted that, when the statistic characteristic of the amplitude of a clutter signal is in accordance with the Rayleigh distribution (many weather clutters and some sea clutters are known to be in accordance with the Rayleigh distribution), the clutter signal is suppressed to have the same amplitude distribution as the receiver noise by LOG/CFAR processing.
In this manner, it is possible to separate target and clutter signals and to suppress only the clutter signal by the Doppler filters 7A to 7N and the CFAR processors 8A to 8N.
Referring to FIG. 1, output signals from the CFAR processors 8A to 8N are input to an automatic target detector 9. The automatic target detector 9 conventionally performs automatic target detection in units of Doppler filter channels for the following reasons. As described above, each clutter signal has an inherent Doppler frequency and its statistical characteristic is different depending on the object contributing to reflection. When the reflecting object is an aggregate of uniform reflection points, such as a rain cloud, it can often be described in accordance with the Rayleigh distribution. However, when the reflecting object is an aggregate of non-uniform reflection points, as in the case of ground clutter, it is possibly in accordance with the Weibull distribution or the Log-Normal distribution. In this manner, the amplitude distribution characteristics of a clutter signal vary depending on the type of clutter, the physical characteristic of the reflecting object, and so on, and thus amplitude distribution characteristics after MDF and LOG/CFAR processings also vary.
When automatic target detection is to be performed, a detection threshold level must be automatically controlled in order to correspond to the amplitude distribution characteristics of various remaining clutter signals after clutter suppression processing and to suppress the probability of erroneously detecting a remaining clutter signal as the target signal at a predetermined level or lower. The signal amplitude distribution characteristics differ from one Doppler filter channel to another in accordance with the presence/absence of clutter and with the type of clutter. Also, the presence/absence and the type of clutter changes from one area to another. Therefore, the automatic threshold control must be independently performed in units of Doppler filter channels.
However, the conventional method of a radar system which comprises an automatic target detection/processing means in units of Doppler filter channels has the following drawbacks. First, a received signal is digitized into a binary signal by a threshold level which is preset in accordance with a remaining clutter level in units of Doppler filter channels. A target is finally discriminated. With this method, an optimally selected video signal cannot be obtained because amplitude information is lost due to quantization process in the stage of the target detector, where the optimally selected video signal is such a signal that preserves amplitude information of target, receiver noise and clutter residue. Amplitude interpolation for target signal should be considered when necessary to reconstruct the partially deteriorated target amplitude due to canceller processing. Therefore, when a malfunction of an automatic target detecting/processing system, a disturbance, or an interference occurs to disable automatic detection, an video allowing visual determination by an operator cannot be provided.
Second, the target detection/processing systems must be provided to coincide in number with the Doppler filter channels, resulting in a large hardware scale.